The invention relates to a low-pressure mercury vapor discharge lamp provided with a discharge vessel,
which discharge vessel surrounds in a gastight manner a discharge space provided with a filling of mercury and a rare gas,
electrodes being arranged in the discharge space for generating and maintaining a discharge in the discharge space, and
an electrode shield surrounding at least one of the electrodes at least substantially.
Mercury is the primary component for an (efficient) generation of ultraviolet (UV) light in mercury vapor discharge lamps. A luminescent layer comprising a luminescent material (for example, a fluorescent powder) may be present on an inside wall of the discharge vessel for the conversion of UV to other wavelengths, for example to UV-B and UV-A for sun tanning purposes (sun couch lamps) or to visible radiation for general lighting purposes. Such discharge lamps are accordingly also referred to as fluorescent lamps. The discharge vessel of a low-pressure mercury vapor discharge lamp is usually circular in shape and there are both elongate and compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a set of comparatively short straight portions of comparatively small diameter, which straight portions are interconnected either by means of bridge portions or by means of curved portions. Compact fluorescent lamps are usually provided with (integrated) lamp caps.
A low-pressure mercury vapor discharge lamp of the kind mentioned in the opening paragraph is known from the English abstract of Japanese patent application JP-A 62-208 536. In the known lamp, the electrode shield surrounding the electrode is cylindrical in shape and is provided with a narrow opening in the direction of the so-called positive column. The known electrode shield is manufactured from glass and is electrically insulated from the electrode. The use of the known electrode shield, also called anode shield or cathode shield, achieves that a portion of the high-energy electrons in the so-called negative glow recombines at an inner surface of the electrode shield, while a further portion of these electrodes diffuses through the opening. As a result of this, the negative glow and the positive column are coupled to one another, and the efficiency of the known discharge lamp is improved.
A disadvantage of the use of the known low-pressure mercury vapor discharge lamp is that its mercury consumption is comparatively high. As a result of this, a comparatively high mercury dose is necessary for the known lamp if a sufficiently long life is to be realized. This is detrimental to the environment in the case of inexpert processing after the end of lamp life.
The invention has for its object to provide a low-pressure mercury vapor discharge lamp which consumes comparatively little mercury.
To achieve this, the electrode shield has the shape of a snail shell.
To achieve a good operation of low-pressure mercury vapor discharge lamps, the electrodes of such discharge lamps comprise not only a material with a high melting point (a widely used metal is tungsten), but also an (emitter) material with a low so-called work function (reduced emission potential) for supplying electrons to the discharge (by emission, cathode function) and for receiving electrons from the discharge (anode function). Known emitter materials with a low work function are, for example, oxides of alkaline earth metals. It is observed that (emitter) material is released during operation of such low-pressure mercury vapor discharge lamps, for example owing to evaporation or sputtering of the alkaline earth metals from the electrode(s). It is found in general that these materials are deposited on an inside wall of the discharge vessel. It is further found that the alkaline earth metals deposited elsewhere in the discharge vessel no longer take any part in the process of light generation. The deposited (emitter) material causes so-called blackening (a greying effect) during lamp life, which is often visible in the form of blackish rings adjacent the electrodes of the low-pressure mercury vapor discharge lamp. Such a blackening is undesirable for aesthetic reasons. Chemical analysis also shows that mercury-containing amalgams are formed on the inside wall in the blackening spots. The quantity of mercury available for the discharge is (gradually) reduced owing to the formation of these mercury amalgams, which adversely affects lamp life. To counteract such a loss of mercury during lamp life, a comparatively high mercury dose is necessary in the lamp, which is undesirable for environmental reasons.
An additional advantage of the use of an electrode shield in the form of a snail shell is that the reduction of the blackening on the inside wall of the discharge vessel improves the lumen output during the useful life of the discharge lamp compared with that of the known discharge lamp.
In the known low-pressure mercury vapor discharge lamp, the mercury absorption in the region around the electrode is reduced by the presence of the cylindrical electrode shield. The presence, however, of the comparatively large opening in the known electrode shield, facing towards the discharge space (the direction of the positive column) still renders it possible for a considerable portion of the emitter material released during the operation of the discharge lamp to be deposited on the inside wall of the discharge vessel.
When an electrode shield in the shape of a snail shell is used in the low-pressure mercury vapor discharge lamp according to the invention, the (emitter material on the) electrode is entirely overshadowed by the electrode shield, as seen in the direction of the discharge space. Such an electrode shield in the shape of a snail shell achieves as it were that the electrode does not have a direct xe2x80x9cviewxe2x80x9d of the inside wall of the discharge vessel, looking in the direction of the discharge space. The measure according to the invention considerably reduces the risk of material emitted by the electrode in the direction of the discharge space during operation becoming deposited on the inside wall of the discharge vessel.
It is particularly favorable when the electrode shield is substantially spiral-shaped in cross-section. Such a spiraling electrode shield provides a good passage for the discharge from the electrode to the discharge space. Furthermore, a spiraling electrode shield is easy to manufacture.
The passage of the discharge from the electrode to the discharge space extends through the revolutions of the spiral-wound electrode shield. Provided the spiral comprises at least one full revolution, the electrode will not xe2x80x9cseexe2x80x9d the wall of the discharge vessel anywhere in the direction to the discharge space, in other words, the risk of emitter material being deposited on the wall of the discharge vessel is considerably reduced.
Preferably the electrode shield has a narrow opening, the size of said opening being adapted so as to allow the electrode to pass through. The advantage of this measure is that the electrode shield can be provided around the electrode in the manufacture of the discharge lamp when the electrode has already been mounted (for example by means of a welding operation) on the so-called mount, which comprises current supply conductors which are passed through end portions of the discharge lamp to outside the discharge vessel.
Preferably at least one electrode comprises an alkaline earth metal which is partly released from the electrode during operation, and the electrode shield comprises a material which reacts with or forms an alloy with the alkaline earth metal originating from the at least one electrode. It is found from experiments that the alkaline earth metals in metallic form form amalgams with mercury, and that oxides of alkaline earth metals do not react with mercury. Thus, alkaline earth metals in the form of, for example, BaO, SrO, Ba3WO6, Sr3WO4, etc., do not form amalgams with mercury, whereas metallic alkaline earth metals do combine with mercury, forming, for example, Baxe2x80x94Hg or Srxe2x80x94Hg amalgams under similar circumstances. The provision of an electrode shield comprising a material which reacts with or forms an alloy with the alkaline earth metal originating from the electrode(s) considerably reduces the risk of mercury amalgamating, so that the mercury remains available for the discharge and the mercury consumption of the discharge lamp is reduced.
The electrode shield preferably comprises an oxide of a material which oxidizes the alkaline earth metal. The mercury consumption of the discharge lamp is reduced in that the alkaline earth metals originating from the electrodes and deposited on the electrode shield change their chemical state from metallic to that of a suitable metal oxide. Suitable materials are oxidic materials having more than one oxidation state, the material not being in its lowest oxidation state. Further suitable materials are materials having an oxygen deficit. Preferably, the alkaline earth metal is barium or strontium, and the oxide is chosen from the group formed by MnO2, TiO2, Fe2O3, In2O3, SnO2, SnO2:Sb, ZrO2, Nb2O5, V2O5, Tb4O7, and ZnO. Contact with metallic alkaline earth metal (from the electrode) will lead to the formation of the corresponding oxide of the alkaline earth metal, i.e. BaO and/or SrO.
The electrode shield itself must not absorb an appreciable quantity of mercury. To this end, the material of the electrode shield comprises, for example, at least an oxide of at least one element from the series formed by magnesium, silicon, aluminum, titanium, zirconium, yttrium, and the rare earths, or the electrode shield is manufactured from a metal, for example iron or titanium. Preferably, the electrode shield is manufactured from a ceramic material comprising aluminium oxide. A particularly suitable electrode shield is one which was manufactured from densely sintered A12O3, also referred to as PCA (PolyCrystalline Alumina). An additional advantage of the use of aluminum oxide is that an electrode shield manufactured from such a material is resistant to comparatively high temperatures ( greater than 250xc2x0 C.). The risk of the (mechanical) strength of the electrode shield being impaired increases at such comparatively high temperatures, whereby the permanence of shape of the electrode shield is adversely affected. Material (i.e. emitter) originating from the electrode(s) and deposited on an electrode shield manufactured from aluminum oxide which is at such a considerably raised temperature is not or substantially not capable of reacting with mercury present in the discharge as a result of this raised temperature, so that the formation of mercury amalgams is at least substantially prevented. The use of a ceramic electrode shield in the shape of a snail shell according to the invention thus serves a dual purpose. On the one hand, it is effectively counteracted that material originating from the electrode(s) is deposited on the inside wall of the discharge vessel, while on the other hand it is counteracted that (emitter) material deposited on the electrode shield forms amalgams with mercury present in the discharge lamp. Preferably, a temperature of the electrode shield is higher than 250xc2x0 C. during operation. An advantage of such a comparatively high temperature is that the electrode shield becomes hotter than in the known lamp, especially in the initial phase, as a result of which any mercury still bound to the electrode shield is released more quickly and easily.
A further advantage of the use of an electrode shield of aluminum oxide in the shape of a snail shell arises in lamps which are operated on a controllable ballast, for example a so-called high-frequency regulating (HFR) dimming ballast, where an excessive evaporation of (emitter) material of the electrode may take place especially at reduced light levels, while the electrode under these conditions is usually given additional heating through the use of a xe2x80x9cbiasxe2x80x9d current. The electrode shield will catch this material and achieve that the formation of amalgams is effectively counteracted. The mercury consumption of the low-pressure mercury vapor discharge lamp is reduced thereby.
The use of a ceramic electrode shield further reduces the reactivity of materials in the electrode shields, i.e. their tendency to react with mercury present in the discharge vessel for forming amalgams (Hgxe2x80x94Ba, Hgxe2x80x94Sr). The use of an electrically insulating material in addition prevents short-circuiting of the lead wires of the electrode(s) and/or short-circuiting of a number of turns of the electrode(s).
The shape of the electrode shield and its positioning with respect to the electrode influence the temperature of the electrode shield. Preferably, an axis of symmetry of the electrode shield is at least substantially parallel to or coincides substantially with the longitudinal axis of the electrode.